Exactly how unusual is our Solar System?

There is only one Superman. One Wonder Woman, one Captain America, one Ironman. All told, a few dozen superheroes. And billions of superhero fans.

What about our Solar System? Are we one-in-a-million like Batman? Or a dime-a-dozen like Batman’s fans? How unique, how special is our Solar System? For the first time in human history, we now can answer that question. We can even put numbers on it.

When we try to put ourselves in a broader context we need to keep in mind what we look like to others, what the Solar System would look like if we searched for it orbiting another star. Imagine an alien living on a planet orbiting a star 10 light years away from the Sun (in Galactic terms, right next door). After a decade or two spent searching for planets orbiting the Sun, this is what those aliens would find:

Unusual_Solar_System.001.jpg

Pretty sparse, isn’t it? No Earth or Mars. No rocky planets at all! No Saturn, no asteroid belt, no ice giants. All the aliens could detect is Jupiter. And, just like those aliens, all that we would detect of the Solar System around another star would be the Sun and Jupiter. We could pinpoint Jupiter’s mass and orbit with decent precision, but that’s all.

Our question then becomes, how common are Sun-Jupiter systems?

Let’s start with the Sun. It turns out that our home star is a little bit unusual but not a lot. This image shows a census of all the stars within 30 light years of the Sun:

Unusual_Solar_System.005.jpg

Distribution of the types of stars within 30 light years of the Sun (21 white dwarfs are not included on the plot). Credit: Franck Selsis, with data from http://www.solstation.com/stars/pc10.htm

Our Sun is a G star. There are 20 G stars within 30 light years out of almost 400 total stars. The vast majority of stars are M stars, also known as “red dwarfs”. These small red stars have much longer lifetimes than G stars but shine much fainter. Among nearby stars, the Sun is modestly weird. If give our definition of a “Sun-like” star some latitude, our star ends up being rare at the 10% level. That is about the fraction of American adults who are vegetarian.

Now let’s turn our attention to Jupiter. To get started, let’s take a look at our current census of extra-solar planets. Here is a plot containing the almost 3,000 planets that have been discovered in the past two decades:

Unusual_Solar_System.003.jpg

The distribution of extra-solar planets in terms of their masses (y axis) and the size of their orbits (x axis). Colors correspond to the detection technique. The Solar System planets are included for comparison. Data from exoplanets.org.

None of the known extra-solar line up with our rocky planets, or with Saturn, Uranus or Neptune. But we already knew this; it’s why we are searching for Sun-Jupiter systems and not complete Solar Systems.

Like the Sun, Jupiters are just a little out of the ordinary. Only 10-15% of Sun-like stars have a gas giant planet like Jupiter with a mass larger than about 50 times Earth’s mass. Another factor that makes our Jupiter more unusual is its orbit. Our Jupiter’s orbit is nearly circular (to be precise, it is an ellipse with an eccentricity of just 5%) and it is more than 5 times larger than Earth’s orbit. Only about 10% of the known gas giant extra-solar planets have orbits wider than Mars’ (which is only 1.5 times larger than Earth’s) that are nearly circular (with eccentricities less than 10%). Putting those together, only about one in a hundred stars like the Sun has a Jupiter like ours. That is a little less likely than being dealt three of a kind in a 5-card poker hand.

There is one more aspect of the Solar System that is unusual. It’s not something about the planets that we have; it’s something that is missing. About half of all stars like the Sun are orbited by at least one “hot super-Earth” planet. These planets are generally Earth-sized or larger, with orbits smaller than Mercury’s around the Sun.   We don’t know why the Solar System is deficient in super-Earths, although one idea suggests that Jupiter may be the reason.

Let’s put the pieces together to calculate how unusual our Solar System is:

10% (the fraction of stars that are like the Sun)

x 10% (the fraction of Sun-like stars with Jupiters)

x 10% (the fraction of Jupiter-like planets with Jupiter-like orbits)

x 50% (the fraction of Sun-like stars with no hot super-Earths)

 

= 0.05% (1 in 2000)

 

This tells us that about one in every 2000 stars in our Galactic neighborhood is a Sun-Jupiter system. Those are about the odds of being picked if you apply to NASA to be an astronaut. But what does this mean? Well, our Solar System is definitely not rare enough to be Batman. Otherwise there would be thousands of Bat-women and men in every big city. It may be unusual enough for a lesser superpower like being able to eat ten hot dogs in one sitting, but not enough for the kind of superpower we like to talk about.

 

What this does mean is that the Solar System is not a dime-a-dozen kind of planetary system. Most systems are different than ours. Most planetary systems orbit puny red dwarfs instead of big yellow G stars like the Sun. Most systems contain super-Earths close to their stars, and if they happen to have a gas giant it tends to be much closer to its Sun or have a much more stretched out orbit than Jupiter’s.

 

But we don’t know how many Sun-Jupiter systems also have Venuses, Earths, Saturns or Marses. And we don’t know how common Earths and Saturns are in systems without Jupiters or systems that orbit red dwarfs. We really want to know whether there is a reason for life-bearing planets to prefer Sun-Jupiter systems. How exactly does the structure of a planetary system affect its ability to host life, if at all? We are still looking for the answers to those questions. And, who knows, maybe our Solar System will end up being Batman after all.

 


This blog post was the basis for an article I wrote for Nautil.us’s blog — see here.

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21 thoughts on “Exactly how unusual is our Solar System?

  1. I wonder how common instabilities like proposed in the Nice model are, and whether preserving low eccentricities of the inner planets is as unlikely in the other planetary system is as unlikely as Kaib and Chambers say it was for ours.

  2. This doesn’t take into account selection bias. Just because half of the systems we’ve found with planets have a hot super Earth doesn’t mean that half the systems out there have them. It just means that finding them is easy. The larger a planet is and the closer it is to the star, the more easy it is to detect.

    • Well, we only actually find super-Earths around much fewer than half of stars. The value of roughly 50% having super-Earths already takes into account a statistical analysis. Basically, 50% or so of stars must have super-Earths close-in to explain the number that is seen.

  3. I love this absurd takeaway thought: From what we can tell so far our solar system is at least as unique as a person who can eat 10 hotdogs in one sitting.

    Everything about that is wonderful and hilarious.

  4. Dear Sean,
    I hope all is well. I just love your “make it interesting” presentation style. To a novice astronomer like me, I find it fascinating that most the Sun’s solar system does not fall within the mainstream of detected planets. This begs the question… Is this due to a limitation in the detection methods that we use? For example; if we looked at our own solar system from afar, could we in fact detect our own planets? Another graph involving the gravitational pull of the detected planets to date, and that of our own solar system on the mother star might give some indication of the limitation of the detection method that we use?

    • HI Scott — thanks! To answer your question, take a look at the last image in the post (the one with the clumps of “hot super-Earths”, “hot Jupiters” and “normal Jupiters”. I included the Solar System planets as well. You can see that none of the exoplanets falls near any of the Solar System planets, except for Jupiter. So, given our current instruments, we’re stuck looking for Sun-Jupiter systems, not complete Solar Systems.

      • Dear Sean,
        Thanks for your reply. I have attached a lyric that I wrote for your thoughts on our current ability to build a telescope with a resolution that will enable us to take a distortion free picture of a half crescent, Earth sized planet orbiting our nearest star neighbours. If feasible, how big would the telescope have to be?
        Best regards,
        Scott McFarlane

        SALUTE TO THE SUN
        All life is the sun’s soul incarnate.
        The cock’s crow leads twilight’s chorus.
        Holding onto love’s last night’s embrace…
        Life moves out of shadow’s wake…
        And salutes the sun on this date.

        Hallelujah. In the morning sun I do see…
        The wonders of star dust surrounding me.
        Ha-lle-lu-jah. Hallelujah. For all to see…
        Hallelujah. Our Mother sun in her glory…
        Ha-lle-lu-jah. Hallelujah. In her glory.

        At the start of day’s journey…
        Life’s pledge rises in cacophony.
        Attuning to her presence in harmony…
        She uplifts the spirits of all to sea…
        As we go forward in life’s lottery.

        Hallelujah. In the mid-day sun I do see…
        The wonders of star dust surrounding me.
        Ha-lle-lu-jah. Hallelujah. For all to see…
        Hallelujah. Our Mother sun in her glory…
        Ha-lle-lu-jah. Hallelujah. In her glory.

        At the end of day’s journey…
        The wolf’s howl leads the dawn chorus.
        Holding onto love’s last day’s embrace…
        Life moves into shadow’s wake…
        And salutes the sun on this date.

        Hallelujah. In the evening sun I do see…
        The wonders of star dust surrounding me.
        Ha-lle-lu-jah. Hallelujah. For all to see…
        Hallelujah. Our Mother sun in her glory…
        Ha-lle-lu-jah. Hallelujah. In her glory.

        With the passing of the sun…
        Let’s not forget the real beauty of this day…
        So with sadness those not passing this way.
        Let’s not forget we’re part of this parade…
        So with gladness we plan a better way.

        Ha-lle-lu-jah. Hallelujah. Ha-lle-lu-jah!

        Life prays to the sun’s soul incarnate…
        Our pledge, dreams on in symphony.
        Attuning to her absence in harmony…
        She settles the spirits of all to sea…
        As we go forward in life’s story.

        Hallelujah. In the dark I do see…
        The wonders of star dust surrounding me.
        Ha-lle-lu-jah. Hallelujah. For all to see…
        Hallelujah. Her sister suns in their glory…
        Ha-lle-lu-jah. Hallelujah. In their glory.

        Ha-lle-lu-jah. Hallelujah. Ha-lle-lu-jah!

  5. Pingback: Super-Earths: breaking the (resonant) chains! | PLANETPLANET

  6. If you want to know just how weird and rare the earth is, how many civilazations would have their moon at the right distance to cover the sun completly 50% of time and annular eclipses 350% of the time. We are one of the rearest planets to have total solar eclipses and have an intellegent species that understands that.

  7. If you want to know just how weird and rare the earth is, how many civilazations would have their moon at the right distance to cover the sun completly 50% of time and annular eclipses 50% of the time. We are one of the rearest planets to have total solar eclipses and have an intellegent species that understands that.

    • This begs the question: which aspects of the Solar System really matter for life and which are coincidences? I don’t see why the Moon and Sun being about the same angular size in the sky has any connection with life. I don’t think that the Moon is important for life in itself either. It does stabilize Earth’s spin axis, but Earth’s spin axis would be stable without the Moon if the Earth spun a bit faster, which it would if not for the Moon….

      • Tidal forces seem (to me at least) likely to have had a major impact. Stirring the pot so to speak. Without the Moon’s influence our climate would be quite different. Whether or not that means that life would have or would have not arisen is unknown but it’s quite likely that life would at least be very different.

      • I don’t think there is any evidence that Earth’s climate is affected by the Moon. Climate models of Earth with different axial tilt find pretty similar climates (although things can be somewhat different).

      • I’m not even talking about axial tilt.

        If the Moon affects ocean tides, and if ocean tides affect ocean currents, and ocean currents affect climate then the Moon definitely affects climate. Granted the degree of those affects is certainly debatable.

        Of course I’m not really knowledgeable enough in the subject TO debate it… so I’m not going to try.
        http://oceanexplorer.noaa.gov/facts/climate.html

      • Sure I totally agree that the Moon affects the Earth’s tides. I just don’t think there is any evidence that a large Moon like ours is a prerequisite for life.

      • Yes you are correct, but how many planets would be in the same state in time if a civilazation was developing. It does not matter to life but it does matter to man and the chances of other civilazations. If we are the only advace life in the galaxy then it is insanely impossible to have this coincidence. It would most likely be an indicater of how common advaced civilazations exist.

  8. Two thoughts.
    1. How can we know which features are important? For example, I met my wife in a french class in college. It turns out I was not supposed to be in that class because I grew up speaking french (when I was little). But I didn’t feel like taking the placement test. Now we live in France. What are the odds of that happening? Basically zero. But it did! And the odds of anyone following their own particular path in life are zero! But they do, of course. So, to get back, did it matter for civilization that we have a moon and eclipses? Or is it just a chance interesting thing?

    2. If we accept that Moons are vital for the development of civilization, then what fraction of Earth-like planets have moons like ours? Probably not that many. We do think they form readily, although we are only 1 for 4 (rocky planets with moons) in the Solar System. The Moon’s orbit would be unstable if Earth was at half its orbital radius around the Sun. So, for any Earths orbiting stars with habitable zones closer-in than 0.5 AU, large moons would be unstable on billion-year timescales or shorter.

    • The real question is not that it is a coincidence, but in a thousand years when we become part of the galactic culture, why no one made this point. Maybe two million civilizations in the galaxy and none of them has this specific coincidence, it will be humiliating to our children that we are that stupid! Yes, we will not know how rare it is until we find many moons around exoplanets, but looking at how different planetary systems there are with the little data that has been gathered seems to indicate it will be quite rare. Take your wife to see a total solar eclipse and see what she thinks!

  9. Pingback: Exobiologias | Kode Substanz

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